EUV Mask and Method for Forming the Same

ABSTRACT

An extreme ultraviolet (EUV) mask can be used in lithography, such as is used in the fabrication of a semiconductor wafer. The EUV mask includes a low thermal expansion material (LTEM) substrate and a reflective multilayer (ML) disposed thereon. A capping layer is disposed on the reflective ML and a patterned absorption layer disposed on the capping layer. The pattern includes an antireflection (ARC) type pattern.

PRIORITY DATA

This application is a continuation application of U.S. application Ser.No. 13/328,166, filed Dec. 16, 2011, which is hereby incorporated byreference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs. Suchscaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,the need to perform higher resolution lithography processes grows. Oneof the leading next-generation lithography techniques is an extremeultraviolet (EUV) lithography. Others include X-Ray lithography, ionbeam projection lithography, and electron-beam projection lithography.

EUV lithography employs scanners using light in the EUV region, having awavelength of about 10-15 nm. Some EUV scanners provide 4× reductionprojection printing, similar to some optical scanners, except that thescanners use reflective rather than refractive optics, (e.g., mirrorsinstead of lenses). EUV scanners provide the desired pattern on anabsorption layer (“EUV” mask absorber) formed on a reflective mask. Anabsorption layer however may not fully absorb the incident radiation anda portion of the incident radiation is reflected through the absorptionlayer. This often results in an inadequate aerial image contrast, whichmay lead to poor pattern profiles and poor resolution, particularly aspattern features continue to decrease in size. It is desired to haveimprovements in this area.

SUMMARY

The present disclosure provides many different embodiments of an EUVmask that provide one or more improvements over the prior art. In oneembodiment, an EUV mask includes a low thermal expansion material (LTEM)substrate and a reflective multilayer (ML) disposed on the LTEMsubstrate. A capping layer is disposed on the reflective ML and apatterned absorption layer disposed on the capping layer. The patternincludes an antireflection (ARC) type pattern.

In another embodiment, an EUV mask includes a LTEM substrate and areflective multilayer (ML) of molybdenum-silicon (Mo/Si) disposed on theLTEM substrate. A ruthenium (Ru) capping layer is disposed on the ML anda patterned low reflectivity tantalum boron nitride (LR-TaBN) absorptionlayer is disposed on the Ru capping layer. The pattern defines aplurality of reflective regions and absorptive regions, and the patterndefines a plurality of ARC trenches in the absorptive region.

In yet another embodiment, an EUV mask includes a LTEM substrate with aconductive chromium nitride (CrN) layer coated at its bottom surface. Areflective multilayer (ML) of forty-pairs of molybdenum-silicon (Mo/Si)is disposed on the LTEM substrate and a ruthenium (Ru) capping layer isdisposed on the ML. A patterned LR-TaBN absorption layer is disposed onthe Ru capping layer. The pattern defines a plurality of reflectiveregions and absorptive regions. The pattern also has an ARC pattern inthe absorptive region and a plurality of ARC trenches. The ARC trencheshave a depth to produce destructive interference among reflected lightrays from the absorptive regions, and display in various pattern in theabsorptive regions in different mask areas.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of a lithography system for implementing oneor more embodiments of the present invention.

FIGS. 2-5 illustrate cross sectional views of various aspects of oneembodiment of an EUV mask at various stages of a lithography processconstructed according to aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Referring to FIG. 1, an EUV lithography system 10 includes a radiationsource 20, a plurality of illumination optics 30, a mask 40 (in thepresent disclosure, the terms of mask, photomask, and reticle are usedto refer to the same item), a plurality of projection optics 50, and atarget 60 such as a semiconductor wafer on a substrate stage. However,other configurations and inclusion or omission of devices may bepossible. In the present embodiment, the radiation source 20 includes asource providing electromagnetic radiation having a wavelength in theEUV range. For example, the radiation source 20 provides EUV radiationhaving a wavelength of approximately 13.5 nm. The illumination optics 30are configured to guide a radiation beam to the mask 40. The mask 40 isa reflective reticle such as described in further detail later below.The mask 40 may be positioned on a reticle chuck. The electromagneticradiation reflected from the mask 40 (e.g., a patterned radiation beam)is collected by the projection optics 50. The projection optics 50 maybe reflective and may include a magnification of less than one (therebyreducing the patterned image included in the radiation). The projectionoptics 50 direct the patterned radiation to the target 60 (e.g., asemiconductor wafer). The target 60 includes a photosensitive layer(e.g., photoresist or resist), which is sensitive to the EUV radiation.The target 60 may be held by a target substrate stage. The targetsubstrate stage provides control of the target substrate position suchthat the image of the reticle is scanned onto the target substrate in arepetitive fashion (though other lithography methods are possible). Thelithography system 10 or portion thereof may include a vacuumatmosphere.

The following description refers to the mask 40 and a mask fabricationprocess. The mask fabrication process includes two steps: a blank maskfabrication process and a mask patterning process. During the blank maskfabrication process, a blank mask is formed by depositing suitablelayers (e.g., multiple reflective layers) on a suitable substrate. Theblank mask is patterned during the mask fabrication process to have adesign of a layer of an integrated circuit (IC) device (or chip). Thepatterned mask is then used to transfer circuit patterns (e.g., thedesign of a layer of an IC device) onto a semiconductor wafer. Thepattern can be transferred over and over onto multiple wafers throughvarious lithography processes. Several masks (for example, a set of 15to 30 masks) may be used to construct a complete IC device.

Various masks are fabricated for being used in various processes. Forexample, an EUV mask in an EUV lithography processes can be used toprint features with smaller critical dimensions (CD) than otherconventional techniques. An unique set of challenges arises from maskingand reflection of EUV radiation. For example, most condensed materialsabsorb at the EUV wavelength, so a reflective mask may be needed for EUVlithography processes.

Referring to FIG. 2, an EUV blank mask 100 comprises a low thermalexpansion material (LTEM) substrate 110. The LTEM substrate 110 mayinclude LTEM glass, quartz, silicon, silicon carbide, black diamond,and/or other low thermal expansion substances known in the art. The LTEMsubstrate 110 serves to minimize image distortion by mask heating. Inthe present embodiment, the LTEM substrate includes materials with a lowdefect level and a smooth surface. In addition, a conductive layer 105may be deposited on the bottom surface of the LTEM substrate 110. Theconductive layer 105 is operable to provide for electrostaticallycoupling the EUV blank mask 100 to a mask chuck. In an embodiment, theconductive layer 105 includes chromium nitride (CrN), though othercompositions are possible.

A reflective multilayer (ML) 120 is disposed over the LTEM substrate110. The reflective ML 120 includes a large number of alternating layersof materials having a high refractive index and a low refractive index.A material having a high refractive index has a tendency to scatter EUVlight and on other hand, a material having a low refractive index has atendency to transmit EUV light. Pairing these two type materialstogether provides a resonant reflectivity. The ML 120 includes aplurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs(e.g., a layer of molybdenum above or below a layer of silicon in eachfilm pair). Alternatively the ML 120 may include molybdenum-beryllium(Mo/Be) film pairs, or any material that is highly reflective at EUVwavelengths can be utilized for the ML 120. The thickness of each layerof the ML 120 depends on the EUV wavelength and the incident angle. Thethickness of the ML 120 is adjusted to achieve a maxim constructiveinterference of the EUV light reflected at each interface and a minimumabsorption of the EUV light by the ML 120. The ML 120 may be selectedsuch that it provides a high reflectivity to a selected radiationtype/wavelength (e.g., reflectivity of 70%). A typical number of filmpairs is 20-80, however any number of film pairs is possible. In anembodiment, the ML 120 includes forty pairs of layers of Mo/Si. EachMo/Si film pair has a thickness of 5-7 nm, with a total thickness of 300nm.

A capping layer 130 is formed on the ML 120 to prevent oxidation of theML during a mask patterning process and an absorber layer repairingprocess. In addition, the capping layer 130 acts as an etch stop in anabsorption layer patterning process. In the present embodiment, thecapping layer 130 has different etch characteristics than the absorptionlayer. The capping layer 130 includes ruthenium (Ru) with a 20-80 nmthickness. Alternatively, the capping layer 130 may include silicondioxide (SiO₂), amorphous carbon or other suitable compositions. A lowtemperature deposition process is often chosen for the capping layer toprevent interdiffusion of the ML 120.

An absorption layer 140 is formed on the capping layer 130. Theabsorption layer 140 preferably absorbs radiation in the EUV wavelengthranges projected onto the patterned EUV mask. The absorption layer 140may include chromium, chromium oxide, titanium nitride, tantalumnitride, tantalum, titanium, or aluminum-copper. The absorption layer140 may be formed of multiple layers. For example, the absorption layer140 is formed by a dual-layer of chromium and tantalum nitride. In thedepicted embodiment, the absorption layer 140 includes low reflectivitytantalum boron nitride (LR-TaBN). In a subsequent etching process,LR-TaBN shows a more anisotropically and a faster etch than chromium.LR-TaBN also shows adequate overetch tolerance, a controllable etchprofile, and a negligible etch bias. The absorption layer 140 may be anysuitable thickness for a given material to achieve an adequateabsorption.

An antireflection (ARC) layer 150 is deposited above the absorptionlayer 140. The ARC layer 150 is configured to reduce a reflection of alithographic radiation having a wavelength shorter than the deepultraviolet (DUV) range from the absorption layer 140 for a DUVinspector. The ARC layer 150 may use compound materials such as TaBO,Cr₂O₃, ITO, SiO₂, SiN, TaO₅, or any suitable material.

Additionally or alternatively, in an embodiment, a buffer layer (notshown) is formed on the capping layer 130 as an etch stop layer forpattering the absorption layer 140 and a sacrificial layer during asubsequent focused ion beam (FIB) defect repair process for theabsorption layer 140. The buffer layer may include silicon dioxide(SiO₂), silicon oxynitride (SiON) or other suitable materials.

One or more of the layers 105, 120, 130, 140 and 150 may be formed byvarious methods, including physical vapor deposition (PVD) process suchas evaporation and DC magnetron sputtering, a plating process such aselectrode-less plating or electroplating, a chemical vapor deposition(CVD) process such as atmospheric pressure CVD (APCVD), low pressure CVD(LPCVD), plasma enhanced CVD (PECVD), or high density plasma CVD (HDPCVD), ion beam deposition, spin-on coating, and/or other methods knownin the art. In the present embodiment, the absorber layer 140 isdeposited by a sputtering deposition technique to achieve an adequatethickness uniformity, with relatively low defects and good adhesion. Thecompositions and/or physical structures of one or more layers describedabove may be selected based upon reflectivity/absorption of theradiation to be used with the mask 100, the stress compatibility withadjacent layers, and/or other criteria known in the art.

Referring to FIG. 3, in the present embodiment, the absorption layer 140is patterned to form the design layout pattern mask 200 on the blankmask 100. The absorption layer 140 is patterned to define a plurality ofspecified absorptive regions, 210, where the absorption layer 140remains, and a plurality of specified reflective regions, 220, where theabsorption layer 140 is removed. An incident light ray 230 from an EUVlithographic light source (not shown) on the mask 200 is reflectedmainly from the interfaces of the ML 120 below the absorption layer 140.The reflected light ray from the ML 120 in the reflective region 220forms a reflected light ray 240. The reflected light ray 240 is apatterned beam, according to the pattern defined by the absorption layer140. The reflected light ray 240 is collected and projected to a targetsubstrate (e.g., a semiconductor wafer) through a projection opticsystem. In the meantime the incident light ray 230 may be reflectedthrough the absorption layer 140 in the absorptive region 210 to form areflected light ray 250. In some cases, the reflected light ray 250 isquite strong in intensity thus the absorptive region 210 may act as areflective region of the EUV mask 200 rather than being an absorptiveregions. Hence, the pattern projected onto the wafer (i.e., the patterndefined by the reflections from the EUV mask 200) may not be a desiredpattern (i.e., the pattern as defined by the reflective regions 220 andthe absorptive regions 210). The pattern projected onto the wafer maysuffer from low aerial image contrast. Additionally, reflection lightforms at a mask border will be a noise source and impact the waferwindow.

In order to reduce or eliminate such undesirable reflections from theabsorptive region 210, an EUV mask 400 can be used instead of the EUVmask 200 discussed above. The difference between the EUV mask 400 and200 is that an antireflection (ARC) pattern is added to the absorptionlayer 140 in the absorptive region 210, as shown in FIG. 4. TheARC-pattern includes a plurality of ARC trenches 410 being formed in theabsorption layer 140 of the absorptive region 210. The reflected lightray 250 is the light ray, which is reflected from the ML 120 travelingthrough a full thickness of the absorption layer 140. Another reflectedlight ray 450 is a light ray, which is reflected from the ML 120traveling through the trench (traveling through a portion of thicknessof the absorption layer 140). The difference in the traveling length(e.g. the depth of the ARC trench 410) of reflected light rays 250 and450 results a phase difference when they interfere with each other.

The depth of the ARC trench 410 is tuned to have a dimension such that adestructive interference is produced between the reflected light ray 250and 450. With the chosen depth of the ARC trenches 410, reflected lightrays 250 and 450 will be opposite in phase with respect to each otherand cancel each other out by destructive interference. Hence,undesirable reflections from absorption layer 140 in the absorptiveregion 210 are reduced. Due to the periodic nature of destructiveinterference, various depths of the ARC trenches 410 can be chosen. Thedepths of the ARC trenches 410 can be chosen differently in differentabsorptive regions 210 of mask areas of the EUV mask 400. In thedepicted embodiment, the depth of the ARC trenches 410 is about 40 nmformed in the LR-TaBN absorption layer 140.

The ARC trenches 410 can be formed with various trench profiles, such asvertical, non-vertical, flat-bottom trench and non-flat-bottom trench.In the depicted embodiment, the ARC trenches 410 are formed in avertical trench profile with a flat bottom. The ARC trenches 410 can bedisplayed in various ARC-patterns, such as a dense line pattern, densehole pattern or other suitable patterns. Different ARC patterns (by ARCtrenches 410) can be used in the absorptive regions 210 of differentmask areas, such as in a mask border area 510 and in a mask main patternarea 520 (as shown in FIG. 5). The ARC trenches 410 and the reflectiveregions 220 can be formed together by a single patterning process or beformed separately by a multiple patterning processes. In the depictedembodiment, the ARC trenches 410 is formed with the reflective regions220 together by a single patterning process.

A patterning process of the absorption layer 140 includes photoresistcoating (e.g., spin-on coating), soft baking, mask aligning, exposure,post-exposure baking, developing the photoresist, rinsing, drying (e.g.,hard baking), other suitable processes, and/or combinations thereof.Alternatively, the photolithography exposing process is implemented orreplaced by other proper methods such as maskless photolithography,electron-beam writing, and/or ion-beam writing. An etching process isfollowed to selectively remove portions of the absorption layer 140 (andthe capping layer 130) to uncover portions of the ML 120 on the LTEMsubstrate 110. The etching process may include dry etching, wet etching,and/or other etching methods. The absorption layer 140 etching processmay not only be chosen to achieve a high resolution for EUV masks butalso to have a tight and uniform distribution of the critical dimension(CD) over the EUV mask 400.

The EUV mask 400 may incorporate other resolution enhancement techniquessuch as an optical proximity correction (OPC). The EUV mask 400 mayundergo a defect repair process by a repair system. The mask repairsystem is a suitable system, such as an e-beam repair system and/or afocused ion beam (FIB) repair system

Based on the discussions above, it can be seen that the presentdisclosure offers the EUV mask 400 with an ARC-pattern to reduce thereflectance in the absorptive regions 210 without increasing thethickness of the absorption layer 140. It provides a better processwindow for an IC fabrication. The ARC-pattern reduces the reflectance inborder areas 510, which simplify the EUV mask process by a singlepatterning process. The ARC-pattern filters out high order diffractionwave by the ARC trenches 410. The ARC-pattern improves aerial imagecontrast between the absorptive regions 210 and reflective region 220.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A mask comprising: a reflective multilayer over a substrate; an absorption layer disposed over the reflective multilayer; and an antireflection layer disposed over the absorption layer, wherein a trench extends through the antireflection layer and terminates in the absorption layer.
 2. The mask of claim 1, wherein the absorption layer includes a tantalum boron nitride material.
 3. The mask of claim 1, further comprising a capping layer disposed between the reflective multilayer and the absorption layer.
 4. The mask of claim 3, wherein the capping layer includes a ruthenium material.
 5. The mask of claim 1, further comprising another trench extending through the antireflection layer and into the absorption layer, wherein the trench has a first depth and the another trench has a second depth that is different than the first depth.
 6. The mask of claim 1, further comprising a conductive layer, wherein the reflective multilayer is disposed over the conductive layer.
 7. The mask of claim 1, further comprising a low thermal expansion material substrate, wherein the reflective multilayer is disposed over the low thermal expansion material substrate.
 8. An extreme ultraviolet (EUV) mask comprising: a low thermal expansion material (LTEM) substrate; a reflective multilayer (ML) of molybdenum-silicon (Mo/Si) disposed on the LTEM substrate; a ruthenium (Ru) capping layer disposed on the ML; a patterned low reflectivity tantalum boron nitride (LR-TaBN) absorption layer disposed on the Ru capping layer, wherein the pattern defines a plurality of reflective regions and absorptive regions, and the pattern includes a plurality of ARC trenches in the absorptive region.
 9. The EUV mask of claim 8, wherein the reflective ML comprises forth pairs of Mo/Si with each Mo/Si film pair being 5-7 nm thick.
 10. The EUV mask of claim 8, wherein the ARC trenches having a depth being able to produce destructive interference among the reflected light rays from the absorptive regions.
 11. The EUV mask of claim 8, wherein the ARC trenches are arranged in a dense hole pattern at a mask border area and a dense line pattern at a mask main pattern area.
 12. The EUV mask of claim 8, wherein one of the ARC trenches extends into the LR-TaBN absorption layer towards the Ru capping layer such that a portion of the LR-TaBN absorption layer extends from a bottommost surface of the trench to a top surface of the Ru capping layer.
 13. The EUV mask of claim 12, wherein another one of the ARC trenches extends into the LR-TaBN absorption layer to the Ru capping layer.
 14. The EUV mask of claim 8, further comprising a conductive layer, wherein the reflective multilayer is disposed over the conductive layer.
 15. The EUV mask of claim 8, further comprising an antireflection layer disposed over LR-TaBN absorption layer.
 16. A method of forming a mask comprising: forming a reflective multilayer over a substrate; forming a capping layer over the reflective multilayer; forming an absorption layer over the capping layer; forming an antireflection layer over the absorption layer; and removing a first portion of the antireflection layer and a first portion of the absorption layer to form a first trench extending through the antireflection layer that terminates in the absorption layer.
 17. The method of claim 16, further comprising removing a second portion of the antireflection layer and a second portion of the absorption layer to form a second trench extending through the antireflection layer that terminates at the capping layer.
 18. The method of claim 16, further comprising removing a second portion of the antireflection layer and a second portion of the absorption layer to form a second trench extending through the antireflection layer and into the absorption layer, wherein the first trench has a first depth and the second trench has a second depth that is different than the first depth.
 19. The method of claim 16, wherein the capping layer includes a ruthenium material, and wherein the absorption layer includes a tantalum boron nitride material.
 20. The method of claim 16, wherein removing the first portion of the absorption layer includes performing an etching process. 